Device and method for controlling slot-based channel system, and slot-based channel access terminal in wireless LAN

ABSTRACT

An apparatus and a method of controlling slot-based channel access by an access point (AP) to manage a network in a wireless local area network (WLAN) environment and a slot-based channel access terminal are disclosed. A slot-based channel access control apparatus of a WLAN system according to an exemplary embodiment determines one of each terminal and a group of terminals as a slot allocation target, allocates a slot defined in a beacon interval to the determined terminal or group to distinguish channel access time, and transmits a beacon including information on the allocated slot.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/686,073, filed Aug. 24, 2017, which is continuation of U.S. patentapplication Ser. No. 14/409,456, filed Dec. 18, 2014, now U.S. Pat. No.9,781,741, which is a U.S. National Stage of PCT/KR2013/005414, filedJun. 19, 2013, which claims priority to Korean Patent Application No.10-2012-0065519, filed Jun. 19, 2012, 10-2012-0075397, filed Jul. 11,2012, 10-2013-0007172, filed Jan. 22, 2013, 10-2013-0049514, filed May2, 2013, and 10-2013-0070170, filed Jun. 19, 2013, in the KoreanIntellectual Property Office.

TECHNICAL FIELD

The present invention relates to an apparatus and a method ofcontrolling slot-based channel access by an access point (AP) formanaging a network in a wireless local area network (WLAN) environmentand a slot-based channel access terminal operating based on the method.

BACKGROUND ART

When too many terminals, for example, stations (STAs) are present in awireless local area network (WLAN), a probability that the STAs collidewith each other increases. When an STA that performs channel accesswithout listening to a beacon or an STA of an overlapping basic serviceset (BSS) or neighboring BSS conducts channel access, the STAs maycompete with an STA allocated a slot through a beacon.

Thus, the STA performs channel access in a distributed coordinationfunction (DCF) mode even in the allocated slot. However, when there arerelatively more STAs the which slots are not allocated than STAs towhich slots are allocated in the DCF, a great number of STAs performchannel access for a short period of time, thus increasing a delay inchannel access.

For example, when an STA corresponding to a bit set to “1” in a trafficindication map (TIM) performs channel access to receive buffered data, adelay may occur. Further, in the presence of a hidden node in an uplink,a collision probability increases.

Also, it is important for a terminal or sensor considering power savingto reduce time to detect a channel by decreasing collisions.

A slot size is determined in advance before a beacon is transmitted.Here, an STA may transmit all data using only a portion of a slot. Whenanother STA uses a remaining period of the slot or unused slot period,network resources may be efficiently used.

DISCLOSURE OF INVENTION Technical Goals

An aspect of the present invention provides a method of allocating aslot to a terminal, for example, a station (STA), by an access point(AP) in a wireless local area network (WLAN). Further, an aspect of thepresent invention is to provide a method of releasing an allocated slotwhen the slot is not used by the STA or includes a remaining time afteruse so that another STA may use an unused slot or an unused period ofthe slot.

Further, another aspect of the present invention provides a method ofallocating a slot for channel access, based on a priority to an STA thatneeds to save power or to transmit urgent data, so that the STA is lesslikely to collide and quickly transmits data.

Technical Solutions

According to an aspect of the present invention, there is provided aslot-based channel access control apparatus in a wireless local areanetwork (WLAN) system according to an exemplary embodiment including acontroller to determine one of each terminal and a group of terminals asa slot allocation target, an allocation unit to allocate a slot definedin a beacon interval to the determined terminal or group to distinguishchannel access time, and a transmission unit to transmit a beaconcomprising information on the allocated slot.

The information on the allocated slot may include allocation controlinformation, an association identification (AID), a start slot ID andallocation duration information.

The information on the allocated slot may include an AID and a startslot ID when the slot is allocated to each terminal and resources aresuccessively allocated to each terminal.

The information on the allocated slot may include a start slot IDaccording to order of being set to “1” bit in a traffic indication map(TIM) when resources are allocated only to terminals set to “1” bit inthe TIM.

The terminals may include a TIM station (STA) that checks the beacon andverifies buffered downlink data or a non-TIM STA that does not verifythe beacon or buffered downlink data of a TIM broadcast frame.

The TIM STA may be a scheduled beacon checking STA that listens to thebeacon and conducts channel access in the allocated slot.

The non-TIM STA may be one of a scheduled active polling STA which isallocated the slot in response to a request of the terminal and conductschannel access in the allocated slot, an unscheduled active polling STAconducting channel access without resource allocation, and a terminalfor saving power by setting up null data packet (NDP) paging.

According to an exemplary embodiment, the apparatus may further includea reception unit to receive, from the non-TIM terminal, whether tosupport a target wake time (TWT) function allocated as wakeup time ofthe non-TIM terminal through an association request frame, wherein theallocation unit may allocate an AID of a TWT STA supporting the TWTfunction to a non-TIM STA group supporting the TWT function.

According to an exemplary embodiment, the apparatus may further includea setup unit to set up a period comprising an entire TWT period to beallocated as restricted access window (RAW) when the beacon intervalcomprises a TWT to be allocated by the allocation unit, wherein thetransmission unit may transmit RAW related information to be included inthe beacon.

The RAW related information may be defined as an RAW parameter setinformation element (RPS IE), and the RPS IE may include RAW groupinformation, RAW start time information, RAW duration information andslot information.

The RAW group information may include a page ID, a block offset and ablock range.

The TIM STA receiving the beacon including the RAW related informationmay not conduct channel access in the RAW.

When the RPS IE includes an indicator showing that the RAW is allocatedto the non-TIM STA, the RAW group information and slot information maybe omitted.

The allocation unit may allocate a TWT identified as a flow identifierto each flow when each terminal may include a plurality of flows withdifferent traffic properties, and allocate a plurality of AIDs to eachterminal.

When the RPS IE includes an indicator showing that the RAW is allocatedto the non-TIM STA and the RAW group information is omitted, the TIM STAlistening to the RPS IE may not access a channel set in a channel (CH)indication field during a time of RAW duration from RAW start time.

When the allocated TWT is included during the time of the RAW durationfrom the RAW start time, a TWT STA listening to the RPS IE may conductchannel access at the allocated TWT.

According to an exemplary embodiment, the apparatus may further includea scheduling unit to schedule an RAW for the unscheduled active pollingSTA in advance, wherein when the reception unit receives a PS-poll framefrom the unscheduled active polling STA, the transmission unit maytransmit RAW related information scheduled in advance to the unscheduledactive polling STA.

The RAW related information may be defined as an RPS IE, and the RPS IEmay include RAW group information, RAW start time information, RAWduration information, an indicator showing that the RAW is allocated tothe non-TIM STA and an indicator showing that the RAW is allocated tothe unscheduled active polling STA.

When a plurality of PAWs includes an overlapping period, the setup unitmay set up a network allocation vector (NAV) or access priority by RAWgroup so that the terminals or group does not conduct channel access inthe overlapping period.

The allocation unit may periodically allocate resources for the non-TIMSTA set to be a RAW, as a periodic RAW (PRAW), to the terminals orgroup, and the transmission unit may transmit a beacon includinginformation on the allocated PRAW.

The information on the PRAW may be defined as an RPS IE, and the RPS IEmay include an indicator showing that the allocated RAW is the PRAW, aPRAW period and PRAW start time information.

The transmission unit may transmit the RPS IE to be included in a longbeacon.

The PRAW start time information may include relative PRAW start timecalculated using an offset that is a number of beacon intervals in whicha first PRAW appears from the long beacon.

The PRAW start time information may include absolute PRAW start timecalculated using a timing synchronization function (TSF).

The allocation unit may allocate the slot to the terminals or groupassociated with a relay STA from the relay STA using a PRAW, and thetransmission unit may transmit a beacon including information on theallocated PRAW.

The information on the allocated PRAW may include an indicator showingthat the PRAW is allocated to a single terminal and an AID of the singleterminal.

The allocation unit may allocate different start times of first PRAWsallocated to a plurality of relay STAs, so that resources are allocatedto different relay STAs through time sharing.

According to an exemplary embodiment, the apparatus may further includea detection unit to detect a channel for data sensing time set in theallocated slot, and a determination unit to determine that the allocatedslot is in an idle state when the channel is not detected for the datasensing time.

The transmission unit may transmit information indicating that theallocated slot is in the idle state to a terminal to which the slot isnot allocated.

The transmission unit may transmit information indicating that theallocated slot is in the idle state, being included in a clear to sendto self (CTS-to-self) frame in an NDP form, to a terminal to which theslot is not allocated.

According to an aspect of the present invention, there is provided aslot-based channel access terminal in a WLAN system including areception unit to receive a beacon comprising information on anallocated slot from an access point (AP), and a channel access unit toconduct channel access in the slot based on the information on theallocated slot.

The reception unit may receive information on an RAW comprisingsuccessively allocated slots from the AP, and the channel access unitmay conduct channel access in the slot based on RAW group information,RAW start time information, RAW duration information and slotinformation comprised in the information on the RAW.

According to an exemplary embodiment, the terminal may further include acontroller to set an NAV of transmission opportunity (TxOP) in the slotbased on data capacity.

A slot-based channel access control method in a WLAN system according toan exemplary embodiment includes determining one of each terminal and agroup of terminals as a slot allocation target, allocating a slotdefined in a beacon interval to the determined terminal or group todistinguish channel access time, and transmitting a beacon comprisinginformation on the allocated slot.

Effects of Invention

The present invention provides a method of allocating a slot to aterminal, for example, a station (STA), by an access point (AP) using arestricted access window (RAW) or periodic RAW (PRAW) in a wirelesslocal area network (WLAN), thereby reducing a delay in channel access ofthe STA allocated the slot. Further, power consumption of the STA maydecrease due to a reduced delay.

Further, the present invention provides a method of releasing anallocated slot when the slot is not used by the STA or includes aremaining time after use so that another STA may use an unused slot or aremaining portion of the slot, thereby improving efficient use ofnetwork resources.

The present invention provides a method of allocating a slot for channelaccess, based on a priority to an STA that needs to save power or totransmit urgent data, so that the STA is less likely to collide toincrease data transmission rate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a wireless local area network (WLAN) environmentwhich includes an access point (AP) and a plurality of terminalsaccording to an exemplary embodiment of the present invention.

FIG. 2 illustrates a slot allocation frame used for a slot-based channelaccess control apparatus of a WLAN system according to an exemplaryembodiment.

FIG. 3 illustrates a restricted access window (RAW) parameter setinformation element (RPS IE) used by the slot-based channel accesscontrol apparatus of the WLAN system according to an exemplaryembodiment of the present invention.

FIG. 4 illustrates a field used by the slot-based channel access controlapparatus of the WLAN system to indicate resources allocated to anon-traffic indication map (TIM) station (STA) according to an exemplaryembodiment of the present invention.

FIG. 5 illustrates an RAW including a target wake time (TWT) set for anSTA in the slot-based channel access control apparatus of the WLANsystem according to an exemplary embodiment of the present invention.

FIG. 6 illustrates that an overlapping period occurs in a plurality ofRAWs in the slot-based channel access control apparatus of the WLANsystem according to an exemplary embodiment of the present invention.

FIG. 7 illustrates allocation of resources to terminals associated witha relay STA in the slot-based channel access control apparatus of theWLAN system according to an exemplary embodiment of the presentinvention.

FIG. 8 is a block diagram illustrating a slot-based channel accesscontrol apparatus in the WLAN system according to an exemplaryembodiment of the present invention.

FIG. 9 is a block diagram illustrating a slot-based channel accessterminal in the WLAN system according to an exemplary embodiment of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures.

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

FIG. 1 illustrates a wireless local area network (WLAN) environmentwhich includes an access point (AP) and a plurality of terminalsaccording to an exemplary embodiment of the present invention.

A power saving mode (PSM) method is defined in a WLAN to reduce powerconsumption of the terminals, for example, STA1, STA2, STA3, STA4, STA5,STA6, STA7, STA8 and STA9. The AP 110 periodically transmits a beaconand transmits existence of a buffered frame to STA1, STA2, STA3, STA4,STA5, STA6, STA7, STA8 and STA9 through a Traffic indication map (TIM)field of the beacon. In the following description, STA1, STA2, STA3,STA4, STA5, STA6, STA7, STA8 and STA9 will be collectively referred toas an STA.

The STA in a sleep mode periodically wakes up to receive a TIM of thebeacon transmitted by the AP 110. When a bit value of the STA in the TIMis “0”, the STA goes back to sleep. When the bit value is “1”, the STAmay need to wake up until a last frame scheduled for transmission in acurrent beacon interval is transmitted. The STA may check a more datafield of a frame header transmitted by the AP 110 to identify a lastframe. In particular, a more data field of “0” indicates a last frame,which means there is no more frame to be transmitted, and thus the STAmay operate in the sleep mode after receiving the last frame.

However, power consumption of the STA in the PSM is determined based onnot only traffic to be transmitted to the STA but also traffic to otherSTAs, since an interruption may occur during data transmission betweenthe AP 110 and the STA in an attempt to transmit data between the AP 110and another STA. When an interruption occurs, it takes a longer time forthe STA to receive all buffered frames, thus increasing powerconsumption of the STA. In this instance, as power consumption riseswith a larger number of STAs, a sensor STA considering operation withlow power needs power saving solutions.

One of the solutions is reducing a number of STAs simultaneouslyconducting channel access by allowing the STAs to conduct channel accessat different times. Here, to assign the access times, the AP 110 maydivide a beacon interval or a smaller window interval into time units,for example, slots, and allocate the slots to the STAs. This method maybe referred to as slot-based channel access in the present invention.

A slot allocated to each STA may be identified through a location of theSTA on a TIM bitmap. The STA verifies a TIM, calculates the location ofthe STA, and conducts channel access from a start point of acorresponding slot. The STA may conduct a channel access operation ineach slot based on time division multiple access (TDMA) or carrier sensemultiple access/collision avoidance (CSMA/CA).

In TDMA where a slot resource is allocated only to a predetermined STA,the predetermined STA uses the slot resource and thus may immediatelyconduct channel access from a beginning of the slot without clearchannel assessment (CCA). In CSMA/CA, since an STA that does not listento a beacon in an overlapping basic service set (OBSS) or BSS, forexample, a non-TIM STA, may conduct channel access, an STA performs CCAeven on a slot of the STA, and then uses CSMA/CA of a WLAN to accesswhen it is verified that the slot of the STA is not in use.

Assuming that a slot is allocated for all STAs belonging to a TIM or aslot is allocated for each group of divided STAs, slot duration may befixed for all STAs or each group. However, since the AP 110 may bufferdifferent sizes of data for STAs and use different channels for theSTAs, it takes different periods of time to transmit data to therespective STAs. Thus, a different number of slots may be needed foreach STA.

The AP 110 may allocate a slot to each STA or a group of STAs andallocate a restricted access window (RAW) or periodic RAW including theslot.

The AP 110 may transmit a TIM indicating existence of buffered data toSTA1, STA2, STA3, STA4, STA5, STA6, STA7, STA8 and STA9 by group.

In an infrastructure mode of the WLAN, the AP 110 may buffer a frame forSTA1, STA2, STA3, STA4, STA5, STA6, STA7, STA8 and STA9 that aresleeping in the PSM.

The AP 110 may receive information on service properties or trafficproperties of each STA from STA1, STA2, STA3, STA4, STA5, STA6, STA7,STA8 and STA9.

The information on the traffic properties may be information on TrafficClass (TCLAS) information used in Institute of Electrical andElectronics Engineers (IEEE) 802.11e standards, while the information onthe service properties may be information on an access category (AC) ofenhanced distributed channel access (EDCA). Alternatively, theinformation on the service properties or traffic properties may bedefined newly, based on applications.

The AP 110 may group terminals having similar service properties ortraffic properties. The AP 110 may STA1 121, STA2 122 and STA3 123having similar service properties or traffic properties into one group120. Also, the AP 110 may STA4 131, STA5 132 and STA6 133 having similarservice properties or traffic properties into one group 130. Further,The AP 110 may STAT 141, STA8 142 and STA5 143 having similar serviceproperties or traffic properties into one single group 140.

The AP 110 may broadcast information on target wake time (TWT) havingdifferent values to the groups 120, 130 and 140 so that the STAs in thegroup 120, 130 and 140 wake up at different times to conduct channelaccess. The STAs in each group may acquire the information on the TWT,wake up based on the information, and conduct channel access.

Moreover, when a number of STAs included in the group 120 is greaterthan a preset level, the AP 110 may generate a subgroup 125 to meet thepreset level. The preset level may be determined based on a kind, anumber and a status of channels supported by the AP 110. Also, thepreset level may be defined in various manners, based on specificservice properties or traffic properties.

For instance, when the preset level is two, the subgroup 125 may includeSTA2 122 and STA3 123. Alternatively, the subgroup 125 may include STA1121 and STA2 122, or STA1 121 and STA3 123. Likewise, when a number ofSTAs included in other groups 130 and 150 is greater than a presetlevel, subgroups 135 and 145 may be generated so as to meet the presetlevel. The subgroup 135 may include STA5 132 and STA6 133. The subgroup145 may include STA8 142 and STA9 143.

Alternatively, when one group 120 includes a plurality of subgroups 125,the same TWT setting but different offset values may be provided for thesubgroups. Thus, STAs in different subgroups of the same group may wakeup at different times to conduct channel access. Consequently, STAsbecome less likely to collide in channel access, resulting in a decreasein power consumption of the STAs.

FIG. 2 illustrates a slot allocation frame used by a slot-based channelaccess control apparatus of a WLAN system according to an exemplaryembodiment of the present invention.

The present invention may provide methods of the AP transmittinginformation on a number of slots needed for each STA and information onthe slots to the STA.

A first method uses a slot allocation frame 210. Information onsuccessive slots allocated to the STA may be reflected in the frame. Anelement ID is an identifier (ID) to identify the slot allocation frame210, and a length field indicates a number of allocation fields 220.

An allocation field 220 may include an allocation control field 230, anassociation ID (AID) field, a start slot ID field and an allocationduration field. The allocation control field 230 is used to indicateproperties of allocation and may include a type and a traffic identifier(TID).

The type is used to indicate whether a slot is allocated to apredetermined STA or group. The group may be, for example, a multi-usermultiple-input multiple-output (MU-MIMO) group. When the type is set to“0”, which means that the slot is allocated to the predetermined STA, anAID of the STA is set in the AID field. When the type is set to “1”,which means that the slot is allocated to the group, a group ID of acorresponding group STA is set in the AID field. For example, 6 bits maybe allocated to the group ID. The TID represents a traffic stream (TS)or traffic category (TC).

The AID field illustrates an ID of the allocated STA. The AID field mayrepresent a group ID or partial AID.

The start slot ID, also referred to as a start slot offset or slot startoffset, indicates an ID of an allocation start slot.

The allocation duration represents a size of an interval in which slotsare allocated and is in a unit of slot.

In a second method, assuming that slot resources are allocated to apredetermined STA, being allocated successively instead of separatelyfrom each other, a time of a start slot of each resource is only used.Here, duration may be calculated as a period prior to a start slot ID ofa subsequent allocated resource. In this case, the allocation field 220may include the AID field and the start slot ID only.

In a third method, resources are allocated only to an STA with a TIM setto “1” bit, in which case the allocation field 220 may include only astart slot ID allocated in order of being set to “1” bit in a TIMbitmap, omitting the AID. Similarly to the second method, slots from astart slot ID to prior to a subsequent start slot ID may be calculatedas a slot allocated to the STA. The slot allocation field 210 includinginformation on an allocated slot may be transmitted to the STA or groupSTA, being included in a beacon as an information element (IE) orconfigured as a separate frame.

A slot may be allocated for transmission of not only downlink (DL) databut also uplink (UL) data.

However, the AP may allocate a slot to an STA that does not listen tothe beacon and is not included in the TIM, for example, a non-TIM STA.

STAs may be classified into the following types.

A first type is a scheduled beacon checking STA that listens to a beaconand conducts channel access in a time period allowed by the AP for theSTA or a group that the STA belongs to.

A second type is a scheduled active polling STA that is allocated a timeperiod for channel access from the AP upon a request of the STA andconducts channel access in the allocated time period. The scheduledactive polling STA may be also referred to as a TWT STA since the STAsets a TWT in advance with the AP and wakes up at the scheduled TWT toconduct channel access.

A third type is an unscheduled active polling STA that always conductschannel access to transmit UL data without resource allocation.

A fourth type is a null data packet (NDP) paging setup STA that sets upNDP paging when setting a TWT, and saves power of the STA, using anexistence of buffered DL data and efficient signaling forsynchronization as an NDP paging frame of an NDP type.

Among the four types of STAs, the first STA corresponds to a TIM STAthat checks a beacon and verifies buffered DL data. The second and thirdSTAs perform active polling without checking a beacon. The fourth STAdoes not listen to a beacon since the STA receives a paging frameoptimized for the STA, instead of checking the beacon broadcasted to allSTAs or buffered data as a TIM element in a TIM broadcast frame. Thus,the other three types of STAs may be non-TIM STAs.

Since an operation of listening to a beacon to verify a TIM consumespower of an STA, the non-TIM STAs wake up without listening to thebeacon, sense a channel, and then immediately transmit UL data to the APor transmit a PS-poll frame or trigger frame to the AP to identify DLdata if there is no UL data.

In order to decrease power consumption while reducing competition withother STAs, the AP may allocate and report information on time to senddata, for example, target wake time as time to wake up and an intervalindicating a time period allocated after wakeup, to a predeterminednon-TIM STA after the non-TIM STA wakes up.

Further, the AP may report the information on the time allocated to thenon-TIM STA to other STAs listening to the beacon so as to restrictchannel access by the other STAs in the time allocated to the non-TIMSTA, thereby reducing competition with STAs accessing the AP around thesame time and thus decreasing power consumption of the non-TIM STA.

Among the non-TIM STAs, an STA supporting a TWT function may set a fieldrepresenting support for the TWT function to a value indicating thatsupport for the TWT function is possible and transmit the field value tothe AP when making an association request to the AP.

When the AP is able to provide a TWT to a corresponding STA, the AP mayreport the TWT to the STA and allocate an AID of the STA to be includedin the non-TIM STA supporting the TWT function. Allocation of the TWTmay be achieved through exchange of management action frames between theAP and the STA after association. When the AID of the STA allocated theTWT does not belong to the non-TIM STA group supporting the TWT afterallocation of the TWT, the AP may reassign the AID of the STA to beincluded in the non-TIM STA group.

The AP may set a time period including an entire period of the allocatedTWT as a RAW and transmit RAW related information to be included in thebeacon to the STAs.

The AP may include time information allocated to the non-TIM STAs in thebeacon and transmit in a TIM form. When successive times are allocatedto the non-TIM STAs, the AP may group the non-TIM STAs and transmit a ULTIM indicating that there is an allocated time for UL to each non-TIMSTA.

Further, in an UL, each STA may be allocated different times or aplurality of slots may be used, and thus the RAW may be used. In thisinstance, an STA listening to a general TIM belongs to a different groupfrom that of the non-TIM STAs, and thus does not access a time periodallocated to the non-TIM STAs.

When times allocated to the non-TIM STAs are not successive or timeallocated to a non-TIM STA is short, allocation of the slot to a groupmay be inefficient. Further, since slot allocation for the non-TIM STAis realized with respect to a fast time period due to a low duty cycleof the STAs and slot allocation for an STA group listening to the TIM isachieved before the beacon is transmitted, it may be necessary tomaintain time for the non-TIM STA allocated in advance.

Here, the AP may allocate overlapping time. Here, when the AP allocatestime overlapping with time allocated for a non-TIM STA to a group, theAP may report whether the times overlap and overlapping time.

The AP converts time information allocated to the non-TIM STA into aslot unit that is a time unit allocated to a group STA. Then, the AP mayreport information on an allocated slot using the aforementionedslot-based allocation method. Here, the AID of the non-TIM STA has adifferent range from that of the AID of the group STA and thus may notbe indicated in the TIM or slot allocation map. However, since thenon-TIM STA operates without listening to the TIM, the AP reports theinformation on the slot allocated to the non-TIM STA to another STAlistening to the TIM so that the other STA does not access the AP. TheAP may reserve a predetermined AID for reporting the information on theallocated slot. Alternatively, as the allocated slot is reserved, the APmay gather unavailable slots and report information on the unavailableslots through a separate indicator.

While the non-TIM STA conducts channel access to the AP, a resource forthe non-TIM STA set through the RAW tends to be periodically allocatedso as to prevent channel access by another STA. A periodic RAW (PRAW)may be defined for the periodically allocated resource.

An RAW parameter set information element (RPS IE) may include anindication field showing that the RAW is the PRAW and a field indicatinga period in which one RAW is repeated. The AP transmits information onthe PRAW being included only in a predetermined beacon, for example, along beacon, thereby reducing overhead of the RPS IE indicating arepeated PRAW. Particularly, a size of a short beacon may be reduced.

The RPS IE may indicate time at which the RAW starts through a RAW starttime field. The RAW start time may be calculated as a time unit (TU)value after transmission of the beacon ends.

A PRAW start time may be calculated directly from the long beacon in aTU or may be also calculated relatively using an offset. The offset maybe determined as a number of beacon intervals in which a first PRAWappears from the long beacon including the RPS IE having PRAW relatedinformation.

The RAW start time indicates time from a target beacon transmission timeof a short beacon in which a PRAW appears first. Thus, time at which thefirst PRAW appears from the long bacon may be calculated byoffset*beacon interval+RAW start time.

A position of the PRAW after the first PRAW may be calculated byoffset*beacon interval+RAW start time+(i−1)*period of PRAW, defining anindex of an i^(th) PRAW as i. The period may be in a TU.

The STA listening to the RPS IE including the PRAW related informationmay determine whether the PRAW is located in the beacon interval fromthe short beacon to access. When a number of beacon intervals (BIs) tothe short beacon to access is j,j*BI<offset*BI+RAW start time+(i−1)*period<(j+1)*BI0<=i<integer(long beacon period/PRAW period).

When there is an i satisfying the above two inequations, the STAlistening to the RPS IE including the PRAW related information does notconduct channel access for an i^(th) PRAW interval with respect to everyi.

However, the TBTT at which the beacon is sent may be delayed when amedium is occupied in advance. Particularly, in a large offset, aplurality of beacons are transmitted, and thus the TBTT may be likely tobe substantially delayed. In this case, the RAW start time calculatedfrom an end of a beacon also changes. However, since the TWT allocatedfor the non-TIM STA is predetermined and the non-TIM STA does not listento the beacon, the non-TIM STA does not recognize the delay in the TBTT.Thus, the RAW start time may need to be expressed as an absolute time,instead of a relative time. For example, the time at which the firstPRAW appears from the long beacon may not be expressed as a relativevalue from the TBTT but use a timing synchronization function (TSF) asan absolute time.

As compared with use of a relative time, using an absolute time involvesan increase in a number of bits of the RPS IE but enables an STAlistening to only a long beacon without listening to a short beacon torecognize an accurate PRAW start time even though a TBTT of a subsequentshort beacon is different from an expected value.

When a relative time is used, the RAW start time is corrected by adifference between an expected TBTT value and an actual time at whichthe short beacon is received.

Next, a method of reusing an allocated slot will be described. When theAP allocates a slot indicating an access time to an STA, the allocatedslot may not be used by the STA or include a remaining part after theSTA uses the allocated slots for transmitting data traffic. Thus, the APmay allow another STA to use the remaining slot for efficient use ofresources in the network.

To reuse the allocated slot, the STA may need to conduct channel accessfrom a start of the allocated slot. A data sensing time parameter may beadded. The AP may sense a channel to determine whether the STA transmitsdata in the allocated slot during a data sensing time. When the channelis not sensed during this time, the AP may determine that the slotallocated to the STA is in an idle state.

Even though two or more STAs simultaneously access a channel to cause acollision, the AP is able to sense occurrence of collisions and thus todetermine that the channel is busy.

When the allocated slot is determined to be in the idle state, the APreports the slot or channel to another STA. A reporting frame is a formof NDP. The NDP may be, for example, a frame that performs the samefunction as the CTS-to-self frame but includes only a preamble.

A CTS ID field may be set as a preamble in a signal (SIG) field. Amedial access control (MAC) address or BSSID to identify the AP may beset in the CTS ID. Further, the CIS ID may include a partial address dueto limitation on bits of the SIG field. Further, when two or morebandwidths are bonded for use, a duplication mode may be used in whichan entire bandwidth is divided into unit bandwidths and the same frameis transmitted in each unit bandwidth. For instance, in 1-MHzduplication, the AP transmits an NDP CTS-to-self frame in a 1-megahertz(MHz) unit in a released channel bandwidth.

An STA performs a channel access operation in a slot as follows. The STAmay conduct channel access in an allocated slot according to apredetermined channel access mode, for example, TDMA or CSMA/CA.

The STA may conduct channel access from a start point of the slot, andan STA that is not allocated a slot but wishes to use a channel may waitfor an NDP CTS-to-self frame to be transmitted from the AP. When the NDPCTS-to-self frame is not received and the channel is in a busy state,the channel is considered to be used by the allocated STA. However, whenthe NDP CTS-to-self frame is received, which means that the channel isreleased, the STA accesses the channel according to CSMA/CA and attemptsto transmit a frame to the AP.

To reduce collisions with hidden nodes, the STA may transmit data afterexchange of RTS-CTS frames. Subsequently, the STA may transmit a PS-pollframe or trigger frame, for example, to retrieve DL data. Also, the STAmay acquire transmission opportunity (TxOP) to transmit UL data andtransmit the UL data in TxOP. With a TxOP limit going beyond theallocated slot to a remaining slot, when a subsequent slot is allocatedto the same STA as that a current slot is allocated to, the STA may setTxOP beyond the current slot. However, when the next slot is allocatedto another STA, the STA may set TxOP to a current slot interval only.

In the NDP frame indicating that the slot or channel is released so thatanother STA may use the allocated slot, the NDP CTS-to-self frame isprovided for illustrative purposes only, without limiting the NDP framethereto. The NDP frame may be defined newly for the same purpose. Here,an indication of a new NDP frame may be set to a reserved bit in the SIGfield.

When the STA allocated the slot wishes to release the using channel asthere is no more data, a channel release indication method is needed.Implicit and explicit methods may be used to indicate that there is nomore data to send.

An implicit method uses a more data field of a frame header. When a DLis in use and the STA transmits a more data field of an acknowledgement(ACK) field set to 0, the AP determines that there is no more data thatthe STA is to send. Another implicit method sets a network allocationvector (NAV) of TxOP considering a data transmission end when an UL isin use and the STA is allowed to set TxOP. The AP may set up a rule suchthat when the STA does not set TxOP until an end of the slot even with aslot interval remaining, the AP determines that the STA returns theallocated slot since there is no more data to transmit.

In an explicit method, the STA explicitly notifies the AP that use ofthe slot terminates by transmitting a contention free (CF)-end frame ornewly defined frame for truncation of TxOP in enhanced distributedchannel access (EDCA).

When the STA returns the allocated slot according to either of theimplicit and explicit methods, the AP transmits, for example, an NDPCTS-to-self frame to notify another STA that the allocated slot isreleased.

FIG. 3 illustrates an RPS IE used by the slot-based channel accesscontrol apparatus of the WLAN system according to an exemplaryembodiment of the present invention.

An RAW frame 310 may include an RPS IE 320. An element ID is an ID toidentify the RAW frame 310, and a length field indicates a number of RPSIEs 320. An RAW related information is defined in the RPS IE 320. TheRPS IE 320 is included in a beacon and may include a plurality of RAWinformation.

The RPS IE 320 may include an RAW group field, an RAW start time field,an RAW duration field, an option field and a slot definition field. Eachfield indicates as follows.

RAW Group Feature Value Interpretation Page ID 2 bits Indicates the pageindex for hierarchical AID (based on hierarchical AID) of the allocatedgroup Block 5 bits Assuming 32 blocks per page, these bits indicate theOffset starting block index of the allocated group Block 5 bitsIndicates the number of blocks (starting from the block Range offset)for the allocated group RAW 8 bits Duration in TU from end of beacontransmission to Start RAW Start time Time RAW TBD Duration of RAW in TUDuration

Option Field and Slot Definition Field Feature Value InterpretationAccess 2 bits Bit 1: Set to 1 if only STA with their TIM bit set to 1restricted are allowed to perform UL transmissions to paged Bit 2: Setto 1 if RAW is reserved for frames with STA only duration smaller thanslot duration, such as PS-Polls/ trigger frames Group/ 1 bits Set to 1if STAs need to wake up at the beginning of Resource the RAW to receivegroup addressed frames such as allocation resource allocation (format ofthe resource allocation frame frame TBD) Slot TBD Include definitionSlot duration signaling Slot assignment to STA Cross boundarytransmissions allowed/not allowed Format is TBD

The RAW group field indicates an AID of an RAW, and accordingly an STAlistening to the beacon including the RPS IE 320 may verify an RAW towhich the STA belongs. The STA may conduct channel access in the RAWbased on start time of the RAW, duration of the RAW and slotinformation. Further, an RAW that the STA does not belong to is foranother group to conduct channel access, channel access in the RAW isbasically restricted.

When wake time of an STA allocated a TWT is determined by the allocatedTWT, the STA operates without checking the beacon and thus does notrefer to the RPS IE 320. The RPS IE 320 is for an STA listening to thebeacon in another group, which prevents the STA in the other group fromaccessing the RAW allocated the TWT, thereby reducing competition withother STAs at the TWT.

A page ID and a block offset of the RPS IE 320 indicate an AID of afirst STA in a group allowed to use the RAW, and a block range indicatesa range of STAs included in the group subsequent to the first STA.

A predetermined STA listening to the beacon including the RPS IE 320 mayrecognize whether access is possible in the RAW. However, in an RAWgenerated by collecting TWTs allocated to non-TIM STAs, each non-TIM STArecognizes time allocated to the non-TIM STA through separate TWT setupwith the AP and does not listen to the beacon. Thus, the beaconincluding the RPS IE 320 is for preventing channel access by a TIM STAreceiving the beacon.

An additional bit may be included in the RPS IE 320 and be used as anindication reporting that the RAW is allocated to the non-TIM STA.

When the additional bit is set, a slot definition of the RPS IE 320 maybe omitted. Accordingly, bits of the RPS IE 320 may be reduced.

Here, when AIDs of STAs allocated TWTs among successive TWTs aregathered in a group in advance, the RAW group field may distinguish theSTAs allocated the TWTs from other STAs.

However, the AP may have burden of dynamically grouping the AIDs of theSTAs allocated the TWTs since grouping is not easy.

Further, when one STA has a plurality of flows with different trafficproperties, the STA may set a separate TWT by each flow with the AP,with each TWT having a different period. Different TWTs of each STA maybe distinguished through a flow identifier in setting the TWTs.

However, since an AID of an STA may need to be continually changed whenAID grouping is necessary for setting an RAW but the STA is generally ina sleep state out of a predetermined time interval from a TWT, AIDreallocation information may not be reported. Further, even throughreallocation information may be reported, it is needed to continuallyreport the AID changing at each TWT to the STA.

To solve such a problem, a plurality of AIDs may be allocated to an STA.Alternatively, a method of protecting an RAW through 1-bit indication,excluding the RAW group field to protect an allocated TWT, with use ofone AID may be used.

Alternatively, in an NDP paging STA, when one flow has a shorter periodthan a beacon interval and checking traffic in each period is needed, aTWT of the flow with the shorter period than the beacon interval is setonly, thereby operating as a TWT STA.

When TWTs of flows with a shorter period than the beacon interval arecollected, STAs allocated the TWTs may include both TIM STAs and non-TIMSTAs. These two groups are generally allocated different pages in theAID field and thus may not be indicated as one RAW group.

In this case, RAWs are set for the two divided groups or an AID of onegroup is reallocated. A plurality of AIDs may be allocated. However,since there is no problem with an operation between the AP and the STAeven with one fixed AID, indicating an RAW with the RAW group field andreporting that the RAW is allocated to TWT STAs or non-TIM STAs using anadditional bit are adequate, instead of changing or adding an AID.

When the same procedure as the TIM STA is notified of the STAs that theRAW is allowed for is conducted for the non-TIM STAs to increase unity,setting a plurality of AIDs for AID grouping is allowed or AIDreallocation is enabled. In this case, STAs allocated the RAW may bereported through the RAW group field, regardless of TIM STAs and non-TIMSTAs.

FIG. 4 illustrates a field used by the slot-based channel access controlapparatus of the WLAN system to indicate resources allocated to anon-TIM STA according to an exemplary embodiment of the presentinvention.

An RPS IE 410 included in an RAW frame 420 of FIG. 4 uses an indicationbit, for example, a non-TIM RAW indication, to protect resources for aTWT STA or non-TIM STA.

When the non-TIM RAW indication is set to “1”, unnecessary informationmay be deleted from an RAW. For example, an RAW slot definition fieldand an option field may be deleted. Further, an RAW group field may bealso deleted. A deletable field may change based on meaning of anadditional bit. When the bit is set to “0”, which means a general RAW,other fields may be still used.

When the additional bit is set to “0”, the STA listening to a beaconoperates in the same manner as a conventional method. However, when thebit is set to “1”, a new operation is defined, which may vary dependingon a category of the STA.

In a first scenario, when the additional bit is set to “1”, the RAWgroup field is deleted. STAs other than a TWT STA or non-TIM STA do notaccess a channel set in a CH indication from RAW start time to RAWduration when the additional bit is set to “1”. The TWT STA checks theRPS IE 410, and operates in a TWT allocated to the TWT STA when theallocated TWT is within a range from the RAW start time to the RAWduration.

In a second scenario, when the bit is set to “1”, the RAW group field isnot deleted. In this instance, when each STA does not belong to an RAWgroup as defined in an RAW, the STA does not conduct channel access in atime interval indicated in the RAW.

An extended method of using an additional bit for an unscheduled activepolling STA will be described. Since an STA may operate by reporting tothe AP that the STA is a non-TIM STA in an association operation and bya permission for non-TIM support from the AP, the AP manages non-TIMSTAs separately by AID grouping.

The AP schedules an RAW for the unscheduled active polling STA inadvance, and notifies the STA of RAW time to conduct data transmissionwhen the unscheduled active polling terminal wakes up and succeeds inchannel access, for example, in receiving a PS-poll frame. Accordingly,collisions caused by access in an interval scheduled for another STA aschannel access time is unpredictable may be reduced.

Further, the RAW for the unscheduled active polling STA enables STAshaving the same service type or STAs operating with similar accesscategories to be gathered and compete with each other, therebyconducting fair channel access.

To protect resources for the unscheduled active polling STA with theRAW, a protection method for TWT STAs may be extended. Unscheduledactive polling STAs are AID-grouped and thus may be expressed accuratelywith the RAW group field. Alternatively, access of a TIM STA listeningto the beacon may be restricted through the additional bit. However,since access of a TWT STA listening to the RPS IE is possible throughthe additional bit, allocation of an additional bit may be needed forthe unscheduled active polling STA to distinguish channel access of theunscheduled active polling STA from that of the TWT STA.

In particular, the additional bit to direct permission for access of theunscheduled active polling STA may be set in the RPS IE. The AP mayallow channel access of the unscheduled active polling terminal throughthe bit, and other fields are not deleted or changed even though the bitis set to “1”.

The AP sets the bit to “0” so as not to allow the unscheduled activepolling STA to access a resource for the TWT STA. Further, to allocate aresource only to the unscheduled active polling STA, the AP sets the bitto “1” and specifies an AID range of the unscheduled active polling STAallowed to access in the RAW group field.

When the resource is allocated to another type of STAs, not for theunscheduled active polling STA, even with the bit set to “1”, it isreported that the unscheduled active polling STA is allowed to accessthe resource for the other type of STAs.

FIG. 5 illustrates an RAW including a TWT set for an STA in theslot-based channel access control apparatus of the WLAN system accordingto an exemplary embodiment of the present invention.

Referring to FIG. 5, the RAW is set to include a TWT interval, so thatonly an STA with a TWT set is allowed to conduct channel access duringan RAW interval.

An STA with a set TWT not included in the RAW interval or other types ofSTAs are not allowed to access the RAW.

FIG. 6 illustrates that an overlapping period occurs in a plurality ofRAWs in the slot-based channel access control apparatus of the WLANsystem according to an exemplary embodiment of the present invention.

In FIG. 6, RAW1 for a TWT and RAW2 for another group are not scheduledin different periods and thus overlap each other. In this instance,access of other groups to RAW1 for the TWT is restricted. However, sinceRAW2 for the group includes RAW1, the group belonging to RAW2 may accessRAW1.

When an RAW for a group overlaps an RAW for another group, a rule is setup such that an NAV is set to restrict access of the other group to theRAW for the group, and accordingly an STA in the group belonging to RAW2does not access RAW1. Alternatively, access priority is set by groupcorresponding to an RAW, and a group with higher priority may be allowedto access an RAW when two RAWs overlap each other.

For example, when a non-TIM STA group with respect to RAW1 including aset TWT has higher priority than other groups, a group belonging to RAW2is restricted to access RAW1. Further, an STA allocable the TWT in RAW1does not listen to a beacon and thus is not restricted to access RAW2.

FIG. 7 illustrates allocation of a resource to terminals associated witha relay STA in the slot-based channel access control apparatus of theWLAN system according to an exemplary embodiment of the presentinvention.

A PRAW may be used not only for protection of a resource allocated to anon-TIM STA but when a resource allocated to a TIM STA in an AID rangeis periodic. In addition, the PRAW may be also used to configure ahierarchical network in which a root AP (Rt-AP) is present and a relaybelonging to a network is used as an AP (R-AP) for another network.

In a beacon interval 710 of an Rt-AP, an R-AP may allocate an RAW toeach RAW group 720. The RAW may include slots 730. A corresponding groupor STA may conduct channel access in a slot based on an AID included inthe RAW.

The Rt-AP may allocate the R-AP resources needed for data exchange withSTAs associated with the Rt-AP. When a plurality of R-AP is present, theresources may be allocated periodically, not in the beacon interval. Inthis case, the PRAW may be used.

In particular, the R-AP may notify each R-AP of resources available fora network of the R-AP in the RAW through a long beacon, particularly thePRAW to reduce overhead.

A bit indicating allocation of the PRAW to a single STA may be defined.When the bit is set, page ID, only an AID may be included in an RPS IE,excluding block offset and block range fields. A partial AID may be usedto identify the R-AP.

Time at which the PRAW appears first may be calculated by the samemethod as used for allocation of the PRAW for the non-TIM STA.

In FIG. 7, RAW start time of the PRAW is start time of each R-AP beacon,and an RAW duration value of the RPS IE is set to include an end of theRAW.

When the PRAW does not start immediately after a long beacon, start timeof the first PRAW may be determined using relative time or absolute timedescribed above.

A resource may be allocated to the R-AP through time sharing by changingstart time of the PRAW. For example, when RAW0 of a beacon “0” of theRt-AP is allocated to R-AP0 and an RAW falling on the same position fromthe Rt-AP beacon and occupying the same interval is allocated to oneafter a beacon “1” of the Rt-AP, for example, RAW4 is allocated toR-AP4, the Rt-AP allocates the PRAW to R-AP0 and RO-AP4. Here, sinceRAWs allocated from the long beacon start at different times even withthe same period, the two R-APs may temporally share resources evenly foruse. When different numbers of STAs are associated with the respectiveR-APs or traffic is different for the R-APs, different resources may beallocated by adjusting the RAW duration of the RPS IE.

FIG. 8 is a block diagram illustrating a slot-based channel accesscontrol apparatus in the WLAN system according to an exemplaryembodiment of the present invention.

Referring to FIG. 8, the slot-based channel access control apparatus 800of the WLAN system according to the present embodiment may include anallocation unit 810, a controller 820, a transmission unit 830, a setupunit 840, a reception unit 850, a scheduling unit 860, a determinationunit 870 and a detection unit 880.

The allocation unit 810 may allocate a slot defined in a beacon intervalto each STA or a group of STAs determined by the controller 820 so as todistinguish channel access times.

The controller 820 may determine one of each STA and the group of STAsas a slot allocation target.

The transmission unit 830 may transmit a beacon including information onan allocated slot.

The information on the allocated slot may include allocation controlinformation, an AID, a start slot ID and allocation durationinformation.

The information on the allocated slot may include only an AID and astart slot ID when slots are allocated to respective STAs and resourcesare successively allocated to the respective STAs.

The information on the allocated slot may include only a start slot IDaccording to order of being set to “1” bit in a TIM when resources areallocated only to STAs set to “1” bit in the TIM.

The STAs may include a TIM STA that checks the received beacon andverifies buffered DL data or a non-TIM STA that does not verify thebeacon or buffered DL data of a TIM broadcast frame.

The TIM STA may be a scheduled beacon checking STA that listens to thebeacon and conducts channel access in the allocated slot.

The non-TIM STA may be one of a scheduled active polling STA, which isallocated a slot in response to a request of the STA and conductschannel access in the allocated slot, an unscheduled active polling STAconducting channel access without resource allocation, and a terminalfor saving power by setting up NDP paging.

The setup unit 840 sets up a period including an entire TWT period to beallocated as RAW when the beacon interval includes a TWT to be allocatedby the allocation unit 810, and the transmission unit 830 may transmitRAW related information to be included in the beacon.

The RAW related information may be defined as an RAW parameter setinformation element (RPS IE), which may include RAW group information,RAW start time information, RAW duration information and slotinformation.

The RAW group information may include a page ID, a block offset and ablock range.

The TIM STA receiving the beacon including the RAW related informationmay not conduct channel access in the RAW.

When the RPS IE includes an indicator showing that the RAW is allocatedto the non-TIM STA, the RAW group information and slot information maybe omitted.

When the RPS IE includes an indicator showing that the RAW is allocatedto the non-TIM STA and the RAW group information is omitted, the TIM STAlistening to the RPS IE does not access a channel set in a channel (CH)indication field during a time of RAW duration from RAW start time.

When the allocated TWT is included during the time of the RAW durationfrom the RAW start time, the TWT STA listening to the RPS IE may conductchannel access at the allocated TWT.

When periods of a plurality of RAWs overlap with each other, the setupunit 840 may set up a network allocation vector (NAV) or access priorityby RAW group so that the STAs or group does not conduct channel accessin an overlapping time.

The reception unit 850 may receive, from the non-TIM STA, whether tosupport a TWT function allocated as wakeup time of the non-TIM STAthrough an association request frame, and the allocation unit 810 mayallocate an AID of the TWT STA supporting the TWT function to a non-TIMSTA group supporting the TWT function.

When the STAs have a plurality of flows with different trafficproperties, the allocation unit 810 may allocate a TWT identified as aflow identifier to each flow and allocate a plurality of AIDs to therespective STAs.

The scheduling unit 860 may schedule a RAW for the unscheduled activepolling STA in advance.

When the reception unit 850 receives a PS-poll frame from theunscheduled active polling STA, the transmission unit 830 may transmitRAW related information scheduled in advance to the unscheduled activepolling STA.

The RAW related information may be defined as an RPS IE, which mayinclude RAW group information, RAW start time information, RAW durationinformation, an indicator showing that the RAW is allocated to thenon-TIM STA and an indicator showing that the RAW is allocated to theunscheduled active polling STA.

The allocation unit 810 may periodically allocate resources for thenon-TIM STA set to be an RAW, as a periodic RAW (PRAW), to the STAs orgroup determined by the controller 820, and the transmission unit 830may transmit a beacon including information on the allocated PRAW.

The information on the PRAW may be defined as an RPS IE, which mayinclude an indicator showing that the allocated RAW is the PRAW, a PRAWperiod and PRAW start time information.

The PRAW start time information may include relative PRAW start timecalculated using an offset that is a number of beacon intervals in whicha first PRAW appears from a long beacon.

The PRAW start time information may include absolute PRAW start timecalculated using a timing synchronization function (TSF).

The transmission unit 830 may transmit the RPS IE to be included in thelong beacon.

The allocation unit 810 may allocate a slot to STAs or a groupassociated with a relay STA from the relay STA using the PRAW, and thetransmission unit 830 may transmit a beacon including information on anallocated PRAW.

The information on the allocated PRAW may include an indicator showingthat the PRAW is allocated to a single STA and an AID of the single STA.

The allocation unit 810 may allocate different start times of firstPRAWs allocated to a plurality of relay STAs, so that resources may beallocated to different relay STAs through time sharing.

The detection unit 880 may detect a channel for data sensing time set inthe allocated slot, and the determination unit 870 may determine thatthe allocated slot is in an idle state when the channel is not detectedfor the data sensing time.

The transmission unit 830 may transmit information indicating that theallocated slot is in the idle state to an STA not allocated a slot.

The transmission unit 830 may transmit the information indicating thatthe allocated slot is in the idle state, which is being included in aclear to send to self (CTS-to-self) frame in an NDP form, to the STA notallocated the slot.

FIG. 9 is a block diagram illustrating a slot-based channel accessterminal in the WLAN system according to an exemplary embodiment of thepresent invention.

Referring to FIG. 9, the slot-based channel access terminal 900 of theWLAN system according to the present embodiment may include a receptionunit 910, a controller 920, a channel access unit 930 and a transmissionunit 940.

The reception unit 910 may receive a beacon including information on anallocated slot from an AP.

The controller 920 may set an NAV of transmission opportunity (TxOP) inthe slot based on data capacity.

The channel access unit 930 may conduct channel access in the slot basedon the information on the allocated slot.

The reception unit 910 may receive information on an RAW includingsuccessively allocated slots from the AP.

The channel access unit 930 may conduct channel access in acorresponding slot based on RAW group information, RAW start timeinformation, RAW duration information and slot information included inthe information on the RAW.

When the information on the RAW includes an RPS IE indicating that theRAW allocated to a non-TIM STA, STAs other than the non-TIM STA may notconduct channel access in the allocated RAW.

When the slot-based channel access terminal 900 of the WLAN system is anon-TIM STA, the transmission unit 940 may transmit a PS-poll frame.

Although a few embodiments of the present invention have been shown anddescribed, the present invention is not limited to the describedembodiments. Instead, it would be appreciated by those skilled in theart that changes may be made to these embodiments without departing fromthe principles and spirit of the invention, the scope of which isdefined by the claims and their equivalents.

The invention claimed is:
 1. A method for channel access of an accesspoint (AP) in a wireless local area network (WLAN), the methodcomprising: sensing whether a channel is idle; and transmitting a NullData Packet (NDP) Clear-To-Send (CTS) frame to a station (STA) to allowthe STA to access the channel when the channel is idle, wherein the NDPCTS frame includes a preamble without a Data field, wherein a Signal(SIG) field included in the preamble includes scheduling information andinformation on at least one of a partial association identifier (AID) ora partial basic service set identifier (BSSID) as a CTS ID(Clear-To-Send Identification), and wherein the STA performs channelaccess when the STA receives the NDP CTS frame.
 2. The method of claim1, wherein the NDP CTS frame is used as a trigger and a synchronization.3. The method of claim 1, wherein the STA transmits a frame to the APafter the STA performs the channel access.
 4. The method of claim 1,wherein a time point when the STA wakes up corresponds to at least oneof a slot boundary of time slots allocated to the STA or a target waketime (TWT) for the STA.
 5. An access point (AP) for channel access in awireless local area network (WLAN), the AP comprising: a reception unit;a transmission unit; and a controller, wherein the controller isconfigured to: sensing whether a channel is idle; and causing thetransmission unit to transmit a Null Data Packet (NDP) Clear-To-Send(CTS) frame to a station (STA) to allow the STA to access the channelwhen the channel is idles, wherein the NDP CTS frame includes a preamblewithout a Data field, wherein a Signal (SIG) field included in thepreamble includes scheduling information and information on at least oneof a partial association identifier (AID) or a partial basic service setidentifier (BSSID) as a CTS ID (Clear-To-Send Identification), andwherein the STA performs channel access when the STA receives the NDPCTS frame.
 6. The AP of claim 5, wherein the NDP CTS frame is used as atrigger and a synchronization.
 7. The AP of claim 5, wherein the STAtransmits a frame to the AP using an allocated slot after the STAperforms the channel access.
 8. The AP of claim 5, wherein a time pointwhen the STA wakes up corresponds to at least one of a slot boundary oftime slots allocated to the STA or a target wake time (TWT) for the STA.